Data transmission circuit for establishing a bidirectional data path in a telephone system

Abstract

A data transmission circuit for use in a telephone system. The telephone system includes a telephone line connected with tip and ring terminals and a subscriber telephone coupled across the tip and ring terminals. The on/hook condition of the telephone is determined by a telephone status monitor circuit which periodically examines the voltage across the tip and ring terminals and produces an output signal when the telephone is in an on/hook condition. Following this output signal, and when data is to be transmitted between a first terminal of the data transmission circuit and the tip and ring terminals, a continuous dc loop is established between the tip and ring terminals and, after a dialtone period, the loop is made and broken in accordance with dialing pulses representing a telephone number of the location to be called. At the termination of the dialing operation, the dc loop is maintained and an ac path is established between the first terminal and the tip and ring terminals for the transmission of data between the first terminal and the tip and ring terminals.

We claim: 
     
       1. A data transmission circuit for establishing a data transmission path between a first terminal and tip and ring terminals of a telephone system, said telephone system including a telephone line connected with the tip and ring terminals and a telephone coupled across the tip and ring terminals and to the telephone line, said tip and ring terminals having a voltage thereacross of a value corresponding to the on/hook condition or off/hook condition of the telephone, said data transmission circuit comprising: status monitor circuit means operative to examine on a frequent basis the voltage across the tip and ring terminals and to produce an output signal when the voltage across the tip and ring terminals has a value corresponding to an on/hook condition of the telephone;   first circuit means adapted when data is to be transmitted between the first terminal and the tip and ring terminals and following an output signal representative of an on/hook condition being produced by the status monitor circuit means to uncouple the telephone from across the tip and ring terminals and to establish a continuous dc loop between the tip and ring terminals to the telephone line and to thereafter make and break the dc loop in accordance with a predetermined telephone number;   second circuit means operative upon termination of the making and breaking of the continuous dc loop by the first circuit means to maintain the continuous dc loop between the tip and ring terminals; and   third circuit means operative after the making and breaking of the continous dc loop to establish an ac data path between the first terminal and the tip and ring terminals for the transmission of data between the first terminal and the tip and ring terminals.   
     
     
       2. A data transmission circuit in accordance with claim 1 wherein the status monitor circuit means comprises: first and second control terminals each adapted to receive a series of pulses;   first and second control means coupled, respectively, to the first and second control terminals and operative in response to the series of pulses at the first and second control terminals to be actuated between first and second operating states in accordance with said pulses;   charging circuit means arranged when the first and second control means are in their first operating states to be placed by the first and second control means in circuit with the tip and ring terminals and when the first and second control means are in their second operating states to be removed by the first and second control means out of circuit with the tip and ring terminals;   said charging circuit means being operative when coupled in circuit with the tip and ring terminals by the first and second control means to be charged to the value of the voltage then present across the tip and ring terminals; and   status circuit means arranged when the first and second control means are in their second operating states to be connected by the first and second control means in circuit with the charging circuit means, said status circuit means being operative when connected with the charging circuit means to examine the value of voltage to which the changing circuit means has been charged and, when the value of the charged voltage corresponds to an on/hook condition of the telephone, to produce an output signal.   
     
     
       3. A data transmission circuit in accordance with claim 2 wherein: the first and second control means comprise: first and second relay means coupled, respectively, to the first and second control terminals and operative to be controlled between first and second energization states by the pulses at the first and second control terminals; and   first and second monitor switch means associated, respectively, with the first and second relay means and operative in response to the first and second relay means being controlled between their first and second energization states to switch between corresponding first and second positions;     said charging circuit means being arranged when the first and second monitor switch means are in their first positions to be placed, together with the first and second monitor switch means, in a series path with the tip and ring terminals and across the tip and ring terminals, and further arranged when the first and second monitor switch means are in their second positions to be removed from the series path to the tip and ring terminals and from across the tip and ring terminals.   
     
     
       4. A data transmission circuit in accordance with claim 3 wherein the status circuit means comprises: circuit means adapted when the first and second monitor switch means are in their second positions to be coupled across the charging circuit means, said circuit means being operative to derive a signal from the voltage to which the charging circuit means has been charged and to compare the value of this derived signal with a reference signal and to produce an output signal when the value of the derived signal bears a predetermined relationship to the value of the reference signal representative of an on/hook condition of the telephone.   
     
     
       5. A data transmission circuit in accordance with claim 4 wherein: the charging circuit means includes a capacitance element.   
     
     
       6. A data transmission circuit in accordance with claim 4 wherein: the charging circuit means includes a series combination of a capacitance element and a resistance element.   
     
     
       7. A data transmission circuit in accordance with claim 1 wherein the first circuit means comprises: a first control terminal adapted to receive a first control signal of a first duration;   a second control terminal adapted to receive dialing signals representative of a predetermined telephone number, said dialing signals being received at the second control terminal during the time of the first control signal and subsequent to the initiation of the first control signal at the first control terminal;   a third control terminal adapted to receive a control signal concurrently with the first control signal received at the first control terminal;   first, second and third control means, coupled, respectively, to the first, second and third control terminals;   said first, second and third control means being operative in response to the control signals received at the first and third control terminals, respectively, and prior to the dialing signals being received at the second control terminal, to establish a continuous dc loop between the tip and ring terminals, and said third control means being further operative in response to the control signal received at the third terminal to uncouple the telephone from across the tip and ring terminals; and   said second control means being further operative in response to dialing signals received at the second control terminal to make and break the aforesaid continuous dc loop in accordance with the telephone number represented by the dialing signals.   
     
     
       8. A data transmission circuit in accordance with claim 7 wherein: the first, second and third control means comprise: first, second and third relay means coupled, respectively, to the first, second and third control terminals and operative to be controlled between first and second energization states by the respective signals received at the first, second and third control terminals; and   first, second and third loop switch means associated, respectively, with the first, second and third relay means and operative in response to the first, second and third relay means being controlled between their first and second energization states to switch between corresponding first and second positions, said first and third loop switch means being operative when in their second positions and said second loop switch means is in its first position to establish a continuous dc loop via the first, second and third loop switch means between the tip and ring terminals, said third loop switch means being further operative when in its second position to uncouple the telephone from across the tip and ring terminals, and said second loop switch means being operative when switched between its first and second positions due to the dialing signals at the second control terminal to respectively break and make the aforesaid continuous dc loop.     
     
     
       9. A data transmission circuit in accordance with claim 8 wherein: the control signal received at the third control terminal extends for a time beyond the duration of the first control signal at the first control terminal and beyond the duration of the dialing signals at the second control terminal;   said first and second loop switch means being operative in response to the termination of the respective signals at the first and second control terminals to switch to their first positions and said third loop switch means being maintained in its second position in response to the control signal at the third control terminal; and wherein the second circuit means comprises:     inductive choke means operative when the first and second loop switch means are in their first positions and the third loop switch means is in its second position to maintain a continuous dc loop between the tip and ring terminals via the inductive choke means, the second loop switch means in its first position and the third loop switch means in its second position.   
     
     
       10. A data transmission circuit for establishing a data transmission path between a first terminal and tip and ring terminals of a telephone system, said telephone system including a telephone line connected with the tip and ring terminals and a telephone coupled across the tip and ring terminals and to the telephone line, said tip and ring terminals having a voltage thereacross of a value corresponding to the on/hook condition or off/hook condition of the telephone, said data transmission circuit comprising: first and second control terminals each adapted to receive a series of pulses;   first and second relay means coupled, respectively, to the first and second control terminals and operative to be controlled between first and second energization states in accordance with the pulses at the first and second control terminals;   first and second monitor switch means associated, respectively, with the first and second relay means and operative in response to the first and second relay means being controlled between their first and second energization states to switch between corresponding first and second positions;   charging circuit means arranged when the first and second monitor switch means are in their first positions to be placed by the first and second monitor switch means in circuit with the tip and ring terminals and when the first and second monitor switch means are in their second positions to be removed by the first and second monitor switch means out of circuit with the tip and ring terminals;   said charging circuit means being operative when coupled in circuit with the tip and ring terminals by the first and second monitor switch means to be charged to the value of the voltage then present across the tip and ring terminals;   status circuit means arranged when the first and second monitor switch means are in their second positions to be connected by the first and second monitor switch means in circuit with the charging circuit means, said status circuit means being operative when connected with the charging circuit means to examine the value of voltage to which the charging circuit means has been charged and, when the value of the charged voltage corresponds to an on/hook condition of the telephone, to produce an output signal;   said first and second terminals being further adapted when data is to be transmitted between the first terminal and the tip and ring terminals and following an output signal being produced by the status circuit means to receive, respectively, a control signal of a first duration and dialing signals representative of a predetermined telephone number, said dialing signals being received at the second control terminal during the time of the control signal at the first terminal and subsequent to the initiation of said control signal;   a third control terminal adapted to receive a control signal concurrently with the aforesaid control signal received at the first control terminal;   third relay means coupled to the third control terminal and having first and second energization states;   said first, second and third relay means being operative in response to the signals at the respective control terminals to be controlled between their first and second energization states; and   first, second and third loop switch means associated, respectively, with the first, second and third relay means and operative in response to the first, second and third relay means switching between their first and second energization states to switch between corresponding first and second positions, said first and third loop switch means being operative when in their second positions and said second loop switch means is in its first position to establish a continuous dc loop via the first, second and third loop switch means between the tip and ring terminals, said third loop switch means being further operative when in its second position to uncouple the telephone from across the tip and ring terminals, and said second loop switch means being operative when switched between its first and second positions due to the dialing signals at the second control terminal to respectively break and make the aforesaid continuous dc loop.   
     
     
       11. A data transmission circuit in accordance with claim 10 wherein: the control signal received at the third control terminal extends for a time beyond the duration of the control signal at the first control terminal and beyond the duration of the dialing signals at the second control terminal;   said first and second loop switch means being operative in response to the termination of the respective signals at the first and second control terminals to operate in their first positions and said third loop switch means being maintained in its second position in response to the control signal at the third control terminal; and wherein the second circuit means comprises:     inductive choke means operative when the first and second loop switch means are in their first positions and the third loop switch means is in its second position to maintain a continuous dc loop between the tip and ring terminals via the inductive choke means, the second loop switch means in its first position and the third loop switch means in its second position.   
     
     
       12. A data transmission circuit in accordance with claim 11 wherein: the charging circuit means is arranged when the first and second monitor switch means are in their first positions to be placed, together with the first and second monitor switch means, in a series path with the tip and ring terminals and across the tip and ring terminals, and further arranged when the first and second monitor switch means are in their second positions to be removed from the series path to the tip and ring terminals and from across the tip and ring terminals.   
     
     
       13. A data transmission circuit in accordance with claim 12 wherein the status circuit means comprises: circuit means arranged when the first and second monitor switch means are in their second positions to be coupled across the charging circuit means, said circuit means being operative to derive a signal from the voltage to which the charging circuit means has been charged and to compare the value of this derived signal with a reference signal and to produce an output signal when the value of the derived signal bears a predetermined relationship to the value of the reference signal representative of an on/hook condition of the telephone.   
     
     
       14. A data transmission circuit in accordance with claim 13 wherein: the charging circuit means includes a capacitance element.   
     
     
       15. A data transmission circuit in accordance with claim 14 wherein: the charging circuit means includes a series combination of a capacitance element and a resistance element.